Electric heating elements are commonly used in Heating, Ventilating and Air-Conditioning systems (HVAC). They are commonly used in the primary duct system to heat air being re-circulated within the conditioned air space and HVAC system, and also to heat air being brought into the conditioned space from the outside ambient air via a fresh-air intake or duct. Numerous control systems have been utilized to detect and energize the heating elements when the air being circulated into or through the HVAC system requires heating. For example, U.S. Pat. No. 6,176,306 to Gault discloses a method and a device for controlling the operation of a heat pump. The Gault heat pump control includes an upstream temperature sensor and a downstream temperature sensor mounted on opposite sides of the heating, refrigerant coil. However, the downstream temperature sensor is still upstream of the auxiliary heating unit (an electrical resistance heater). The thermistors T1 and T2 (which correspond to the sensors) sense the temperature on both sides of the heating, refrigerant coil and the operational amplifier calculates the difference. That difference is adjustable to establish the set point by means of variable resistors. If the temperature difference on either side of the coil is below a predetermined threshold, a relay is energized causing the auxiliary heater to turn on. Gault '306 does not show the temperature sensors at equidistant positions on either side, upstream and downstream, from the electric heating element.
U.S. Pat. No. 4,607,787 to Rogers discloses an electronic control and method for increasing efficiency of heating in an HVAC system. Rogers shows, in FIG. 2, temperature sensors upstream and downstream of the HVAC unit. These temperature sensors are utilized to control the HVAC system to make operation more efficient. For heating, the program cycles the HVAC's burner “on” until the temperature differential between the supply and return ducts reaches a first percentage (e.g., 85%) of the rise in a preset period of time (e.g., 5 minutes) as recorded previously during monitoring, and “off” until the temperature differential between the supply and return ducts reaches a second percentage (e.g., 60%), lower than the aforementioned first percentage of the rise. Rogers '787 does not discuss positioning the sensors equidistant away from a heating element.